In the modern information society, the semiconductor is used in all sorts of things. For example, not to speak of the personal computer and the mobile phone, the semiconductor is used in all of digital consumer electronics such as the flat panel display, the HDD DVD recorder, and the digital video camera. As described above, the trend toward the so-called ubiquitous society in which the information processing spreads to all parts of our society beyond the old framework knows no bounds, and this trend will be further accelerated in the future. As the technology serving as the basis thereof, the mere development of the software is insufficient, and the development of new device technology suitable for the various using environments is indispensable more than ever. For example, for the achievement of the electronic paper which can be flexibly bent like a newspaper and a book, a new technology quite different from the conventional one, that is, the technology for fabricating a transistor on a plastic substrate is required. In another example, in order to perform inventory management of all commodities and to calculate the price of many purchased goods in a moment in a supermarket or the like, it is necessary to fabricate a large amount of the RF-ID (Radio-Frequency Identification) tags capable of transmitting the information through radio waves at low cost.
In addition, in another example, in order to realize a display fabricated on the whole area of the wall, a method for forming transistors on a large area by using a printing technology is required. If this can be realized, the sense of reality as if traveling all over the world is no longer a fantasy even at home. Also, the wearable transistor which can be flexibly bent and can wear well against water and wind is indispensable for fabricating the audio player on the clothes. If the wearable transistors can be highly integrated at low cost, for example, the T-shirt having a moving image thereon is realized. Further, the transistors are embedded not only in the clothes but also in the eyeglasses and ultimately in the contact lenses so as to display the information images thereon. As described above, the transistors which can be integrated on an intended substrate and on a wall are called the flexible transistors. The flexible transistor is expected to be applied to the entirely new field as described above.
The organic transistor is a representative of the flexible transistor. The organic transistor is a field effect transistor in which an organic material such as polythiophene and pentacene is used as the channel material. The organic transistors can be fabricated on a plastic substrate by a printing method such as the inkjet printer and the roll-to-roll printing using the organic semiconductor in a solution state.
However, the performance of the organic semiconductor required in the expected application field is difficult to be achieved due to its low mobility. Although the mobility of the organic semiconductor is remarkably increased owing to the advancement in the material development in recent years, even the single crystal pentacene fabricated by the deposition method has the mobility of only about 1 cm2/Vs. In addition, when forming it by the deposition method, the organic semiconductor is adhered on the whole substrate surface, and the device isolation becomes difficult. Therefore, the deposition method is not suitable for the method of forming the transistors on a flexible plastic substrate. The transistors formed by a printing method which is more suitable than the deposition method is also under development. However, in the case of the printing method, since the single crystallization of the organic semiconductor is difficult, the mobility is reduced by one or two orders of magnitude and is about 0.1 cm2/Vs at most. On the other hand, when the organic EL (Electro-Luminescence) material is used as the display element of the electronic paper, the mobility of about 10 cm2/Vs is required. Also, the mobility of about 10 cm2/Vs is required even in the case where the frequency of about 13.56 MHz band is used for the RF-ID tag. Therefore, it is difficult to satisfy the required performance in the expected application field by the mobility of the organic semiconductor.
The low mobility of the organic semiconductor is derived from the basic characteristics of the material. More specifically, since the coupling between the carrier (the electron or the hole) and the phonon (the lattice vibration) is strong in the organic semiconductor and the carriers are strongly scattered, the mobility is low. In addition, the strong mutual coupling between the electron (hole) and lattice forms a quasiparticle such as polaron and bipolaron and the effective mass of the carriers is increased. As a result, the mobility is decreased due to the increase of the effective mass. This is fundamentally related to the fact that the heavy mass is difficult to move. Therefore, the solution of the problem is difficult. Consequently, even if the organic semiconductor in a single crystal state is formed, the mobility of about 10 cm2/Vs is very difficult to be achieved. Furthermore, the single crystal state is an ideal state and the single crystallization of the entire organic semiconductor is difficult when practically using the organic semiconductor. In the amorphous state and the polycrystalline state other than the single crystal state, the large resistance is caused when the carriers passes through the grain boundary. Therefore, the mobility is lower than that of the single crystal state. This is the common problem in the semiconductor fabricated by the various deposition method and solution method. For example, polycrystalline Si is used for the transistor which is used for driving the liquid crystal display, and the mobility of the polycrystalline Si is one order of magnitude lower than the mobility of single crystal Si used for the LSI. As described above, since the mobility of the organic semiconductor is low due to the unique characteristics of the material and the fabrication method, it is difficult to practically use the organic semiconductor. The organic semiconductor has not only the problem of the lower mobility but also the problem of low reliability, that is, the device characteristics are rapidly degraded in the air. In view of the current situation of the organic semiconductor and the expectation for the flexible transistor, the attempt to realize the flexible transistor by using the inorganic semiconductor as the channel material without using the organic semiconductor has been made.
As a representative example, the fabrication of a polycrystalline silicon chip by the transfer method is known. In this method, the chip in which the transistors using polycrystalline silicon as a channel material are integrated is fabricated on a glass substrate, and then, the chip is separated from the glass substrate and attached to a plastic substrate. According to this transfer method, the same performance as the transistor using the polycrystalline silicon fabricated on the glass substrate can be realized by a chip fabricated on the plastic substrate. When integrating the elements on a flexible plastic substrate, the temperature cannot be increased so high. However, in this transfer method, since the polycrystalline silicon transistor can be fabricated on the glass substrate, a relatively high-temperature process of about 500° C. can be used. As a result, the channel mobility of the transistor of about 10 cm2/Vs can be achieved. However, in the operation for fabricating the polycrystalline silicon transistor on the glass substrate, since the step using lithography is performed many times, the process cost is very high. The increase of the process cost is fatal problem from the viewpoint of the application to the flexible transistor.
For example, for the full-scale spread of the electronic paper, it is necessary to fabricate the electronic paper at the low cost equal to that of the paper. Also, the devices for the RF-ID tags must be fabricated by the process at low cost enough to be disposable. Therefore, the flexible transistors are preferably fabricated by the printing method not by the transfer method. In addition, in the application to a large area, that is, in the case of forming a display device on a whole area of a wall, since there is the limitation in the transfer method in which the transistors are formed on a glass substrate, it is quite difficult to form a display device on a whole area of a wall by the transfer method. As described above, the polycrystalline silicon chip on the plastic substrate by the transfer method is superior to the organic semiconductor in performance. However, it cannot sufficiently satisfy the requirements for the flexible transistor in terms of the cost and the area enlargement. Further, since it uses the polycrystalline silicon, the performance equal to that of the LSI using the single crystal silicon cannot be achieved.
As another method of using the inorganic semiconductor, the method of using a silicon nanowire as a channel material is known. In Nature vol. 425, p. 274 (2003), the field effect transistor using the silicon nanowire on the plastic substrate is disclosed. The silicon nanowire indicates the wire-shaped single crystal silicon with a diameter of several nm to several tens nm and a length of several tens μm or more. The silicon nanowire can be dispersed into a solution by using the surface active agent. As a result, the field effect transistors using the silicon nanowire can be integrated on the plastic substrate by the printing method. Since the crystal structure of the silicon nanowire is the single crystal, the channel mobility of the field effect transistor using the silicon nanowire is about 100 cm2/Vs which is almost equal to that of the single crystal silicon used in the standard LSI. Therefore, the field effect transistor using the silicon nanowire can achieve the high mobility required in the flexible transistor expected for various applications, and thus, it can be said that the field effect transistor using the silicon nanowire is a quite promising device. In addition, for the practical application of the field effect transistor using the silicon nanowire, the knowledge of the silicon transistor developed in the conventional LSI can be utilized for the new application of the flexible transistor.
The silicon nanowire has the other advantages as follows. As is well known in the art, although the silicon substrate used in the standard LSI has a thickness of at least about 500 μm, the region effectively used as the channel of the transistor is at most 100 nm or less, and only the region of several nm from the substrate surface is used as the channel inversion layer. In other words, most silicon substrate is used as a mere supporting substrate. If the silicon layer necessary for forming the channel is about 5 nm, the region effectively used is only one-hundred thousandth. Meanwhile, in the case of the silicon nanowire, the flexible plastic substrate can be used for the supporting substrate, and the silicon necessary for forming the channel is effectively utilized in the transistor using the silicon nanowire with a diameter of several nm to several tens nm. Therefore, it can be said that the transistor using the silicon nanowire is an attractive device in terms of the effective use of the resource.
For fabricating the silicon nanowire, the method of making full use of the nano technology is used. First, metal nanoparticles such as gold and silver with a diameter of about several nm are prepared, and a sample in which the nanoparticles are dispersed on a substrate such as glass is prepared. Next, when the sample is placed in the atmosphere of monosilane gas by using the chemical vapor deposition method, the single crystal silicon nanowire is grown around the metal nanoparticles. As described above, the nanometer scale material, for example, the silicon nanowire can be collected from the chemical reaction at the atomic and molecular level by using the so-called bottom-up process. Since macro devices such as the transistor and the integrated circuit are assembled eventually, the transistor using the silicon nanowire can be regarded as the combination of the bottom-up process and the top-down process.
The above-described field effect transistor using the nanowire structure is not limited to that using silicon as the nanowire material. Actually, even in the case where the nanowire made of germanium or compound semiconductor such as InP is used as the channel material, the field effect transistor can be operated similarly to the case of using the silicon nanowire. Also, the one-dimensional wire structure made of inorganic material is not always necessary. For example, the mobility of the transistor is high also in the case where the carbon nanotube which is a pure organic material is used as the channel material. If a large amount of high-quality carbon nanotube can be obtained in the future, the flexible device using the carbon nanotube can be fabricated in the same process as that of the silicon nanowire.
Also, the wire structure is not always required to have the nanometer scale diameter, and it is also preferable to use the top-down process without using the bottom-up process. Applied Physics Letter, Vol. 84, p. 5398 (2004) discloses the method of forming the silicon wire, in which a silicon layer of an SOI (Silicon-On-Insulator) substrate is processed into a wire shape with a width of several μm and length of several tens μm by using the lithography, and then, SiO2 of the BOX layer is removed by using hydrofluoric acid so as to lift-off the silicon wire. The silicon wires fabricated in the top-down process as described above are wider than the silicon nanowires fabricated in the bottom-up process in many cases. However, the channel mobility is determined based on the wire material and the wire crystallinity and not dependent on the wire width. Therefore, the sufficient performance as the flexible device can be achieved in both cases.